AbstractThis paper considers an asymptotic distribution for an estimate $\hat{S}_{pk}$ of the process yield index $S_{pk}$ proposed by Boyles (1994). The asymptotic distribution of $\hat{S}_{pk}$ is useful in statistical inferences for $S_{pk}$. An illustrative example is given for hypothesis testing and for interval estimation on the yield index $S_{pk}$. Copyright © 2002 John Wiley & Sons, Ltd.